Catalytic hydrocracking is a petroleum refining process which is of increasing world-wide importance due to the continued need for conversion of low quality feedstocks to gasoline products.
The catalysts used in these hydrocracking processes are dual functional types, consisting of a hydrogenation component such as a Group VIII noble metal or a combination of Group VIII (Ni,Co) and Group VIB (Mo, W) metals, in combination with a solid acid catalyst, such as the LZY-B2 or LZ-10 zeolites, amorphous silica-alumina gel, transition aluminas or aluminosilicates. The latter components act as acidic cracking catalysts and they may also act as support for the metal components.
Of the solid acid components, it is generally understood that aluminosilicate zeolites are the most active in the sense that they convert the highest fraction of feedstock to lower boiling products under comparable operating conditions. Activity, however, is only one of three essential properties of a mid-barrel or gasoline hydrocracking catalyst. The other two properties are selectivity to produce the desired products (i.e., gasoline) exclusively, and stability, which is a measure of the useful operating life of the catalyst. It has been found that the high activity of strong acid zeolite catalysts does not compensate for their poor selectivity for turbine and diesel oil, and, accordingly, no commercial maximum mid-barrel catalyst utilizes strong acid zeolites as the principal acid cracking component. Instead, this function is provided either by amorphous compositions, such as silica-aluminas derived from silica-alumina gels, or by the mild acid LZ-10 zeolite, UHP-Y zeolite described in U.S. Pat. No. 4,401,556, which shows much higher selectivity and lower activity than strong acid zeolites.
The chemistry of gasoline and mid-barrel hydrocracking processes is significantly different. In gasoline hydrocracking, multiple fragmentation of the feed molecules is required. In mid-barrel hydrocracking, on the other hand, the average feedstock molecule should be split only once and very near the center of the molecule in order to maximize the mid-barrel fraction, and thereby minimize the production of light hydrocarbons, such as C.sub.1 -C.sub.4 and gasoline. The zeolite component of catalysts employed for gasoline hydrocracking is a strong-acid zeolite, such as LZY-82, REY or LZ-210 zeolite. In the production of diesel and turbine fuels multiple chain branching and multiple cracking are undesirable, consequently, weak or mild acid catalysts are required which yield considerably less isomerization and less multiple fragmentation.
The principal prior art of which the applicants are aware are the following:
U.S. Pat. No. 3,691,099, which issued to Dean A. Young, teaches the acid treatment of zeolite (and other Al-containing refractory oxides}. The patent teaches that salt/acid combination improves soda removal and "acidity" characteristics of Y zeolite. This patent is directed to the acid treatment of NaY and NH.sub.4 Y, not to a stabilized Y. Also, a bulk SiO.sub.2 /Al.sub.2 O.sub.3 of 6-12 is reported, but, a.sub.o measurements indicate that framework SiO.sub.2 /Al.sub.2 O.sub.3 was not substantially altered. PA0 U.S. Pat. No. 3,383,169, which issued to Dean A. Young, teaches low pH ammonium exchange of NaY to improve exchange efficiency (salt usage). PA0 U.S. Pat. No. 4,415,439, which issued to Robert L. Chiang, teaches that calcined NH.sub.4 Y can be treated with acid-aluminum salt solution to yield an improved octane FCC gasoline catalyst. PA0 U.S. Pat. No. 3,130,006, which issued to Jule A. Rabo et al, teaches acid or ammonium treatment of Y or L zeolite prior to calcination to produce "decationized" zeolites for use in hydrocracking. PA0 U.S. Pat. No. 3,887,630, which issued to John W. Ward, teaches stabilized Y zeolite for isomerization of alkyl aromatics and is directed to the basic Y-52 to Y-82 processing. PA0 U.S. Pat. No. 4,093,560, which issued to George T. Kerr et al, teaches acid-treatment of NaY or NH.sub.4 Y to obtain crystalline dealuminated products. Acid-treatment requires days to accomplish dealumination. This patent teaches the required characteristics of the acid component (ionization constants) for controlled dealumination. Rare earth is incorporated in an FCC matrix after acid treatment of Y.